1. Field of the Invention
The present invention relates to an optical wavelength converter for converting an input optical pulse signal having a first wavelength into an output optical pulse signal having a second wavelength which is different from said first wavelength.
2. Description of the Related Art
Such an optical wavelength converter can be preferably used as a resource in the wavelength division multiplex (WDM) optical communication system. In accordance with recent abrupt progress in digital communication systems, the development of the WDM optical communication system has been strongly required. This WDM optical communication system requires an optical wavelength converter in order to utilize channels in an efficient manner by channel switching. Such an optical wavelength converter has been known. For instance, a XGM type optical wavelength converter utilizing cross gain modulation has been proposed. In the XGM type optical wavelength converter, an intensity-modulated input optical signal having a wavelength xcex1 and an optical signal having a wavelength xcex2 and a constant amplitude are supplied to a semiconductor optical amplifier, and a polarity-inverted output optical signal having a wavelength xcex2 is produced by utilizing a difference in gain for optical power impinging upon the semiconductor optical amplifier.
There has been further proposed a XPM optical type optical wavelength converter. A conventional type optical wavelength converter utilizes the principle of the Mach-Zehnder type interferometer.
In the XPM optical wavelength converter, an input side waveguide upon which an input optical signal having a wavelength xcex1 is divided into two waveguides, a semiconductor optical amplifier is arranged in one of the waveguides, and these two waveguides are set to be in-phase for light having a wavelength xcex2 to be modulated. When an input optical signal having a wavelength xcex1 and an optical signal having a wavelength xcex2 and a constant amplitude propagate, there is produced a phase difference of a half wavelength between the two waveguides due to the function of the input optical signal. By utilizing this phase difference, an inverted optical output having the wavelength xcex2 is generated.
In the known XGM type optical wavelength converter, the output optical signal having a wavelength xcex2 produced in response to the input optical signal having a wavelength xcex1 has a rather small gain. Therefore, a zero level of the output optical signal deviates from a real zero. Consequently, the extinction ratio of this optical wavelength converter is small.
In the above mentioned XPM type optical wavelength converter, although it is possible to obtain a sufficiently large extinction ratio, since it reveals a periodical response, an extremely larger tolerance is required for the device length. Therefore, a through-put of the known XPM type optical wavelength converter.
Furthermore, the above mentioned known optical wavelength converter is relatively large in size. That is to say, the typical size of the known optical wavelength converter is not smaller than several to ten millimeters, and thus it is practically difficult to integrate it as a single chip.
The present invention has for its object to provide a novel and useful optical wavelength converter which can avoid or at least mitigate the above explained problems of the known optical wavelength converters and can have a large extinction ratio, can operate even in a digital fashion, and can be manufactured easily.
It is another object of the invention to provide an optical wavelength converter which can be small in size and can be integrated as a single chip.
According to the invention, an optical wavelength converter for converting an input optical pulse signal having a first wavelength into an output optical pulse signal having a second wavelength which is different from said first wavelength comprises:
a first waveguide constructed by an active waveguide and receiving the input optical signal having the first wavelength, a propagation constant of said first waveguide being changed in accordance with the input optical signal; and
a second waveguide arranged in parallel with said first waveguide to partially overlap with said first waveguide such that the first and second waveguides are optically coupled with each other to such an extent that evanescent light of light propagating along one of the first and second waveguides extends into the other waveguide;
wherein said first and second waveguides have a waveguide length L which is substantially equal to a coupling length at which a power transition of an optical signal propagating along the first waveguide into the second waveguide becomes maximum; and
said first and second waveguides are constructed such that a propagation constant difference xcex94xcex2 between the first and the second waveguides in a case that only an optical signal having the second wavelength propagates along said first waveguide is smaller than a propagation constant difference xcex94xcex2 between the first and the second waveguides when both the input optical signal having the first wavelength and the optical signal having the second wavelength propagate along the first waveguide.
According to the invention, said optically coupled first and second waveguides are arranged in parallel with each other and are constructed such that the waveguide length of these first and second waveguides becomes equal to the coupling length. Then, the two waveguides are coupled with each other and constitute an optical coupler in which a light wave propagating along one of the waveguides can be transferred or shifted into the other waveguide. In this case, a transition ratio of optical power is dependent not only upon the waveguide length, but also upon a difference in a propagation constant difference xcex94xcex2 between these two waveguides. When the propagation constant difference xcex94xcex2 decreases, the optical power transition ratio becomes high, and when the propagation constant difference xcex94xcex2 is increased, the optical power transition ratio becomes extremely small. Under a given condition, the optical power transition ratio can be substantially zero. Therefore, by controlling the propagation constant difference xcex94xcex2 between the two waveguides, it is possible to control the condition of the optical coupling between the waveguides in a digital fashion. The present invention is based on such a recognition and at least one of the two waveguides is constructed by the active waveguide in which the propagation constant is changed in accordance with the input optical signal. Therefore, by controlling the propagation constant of the active waveguide, the propagation constant difference xcex94xcex2 between the waveguides can be adjusted to control the optical coupling condition between the waveguides.
According to the invention, the active waveguide may be constructed by a semiconductor optical amplifier. In the semi-conductor optical amplifier, the refractive index of an active layer is changed in accordance with the amount of carriers which are injected into the active layer and are stored therein. When the optical power of the input optical signal is high, the amount of carriers which are consumed by the amplifying function becomes large and the amount of carriers stored in the active layer is decreased, and therefore the refractive index of the waveguide is relatively increased. Contrary to this, when the optical power of the input optical signal is low, the amount of consumed carriers is decreased and the amount of carriers stored in the active layer is relatively increased, and thus the refractive index is decreased. When the refractive index of the waveguide is changed, the propagation constant of the waveguide is also changed. According to the invention, such an active function of the semiconductor optical amplifier is utilized to control the propagation constant of the waveguide through the change in the refractive index of the waveguide in accordance with the optical power of the input optical signal. That is to say, the externally supplied optical signal can be effectively used as a control signal for controlling the propagation constant of the waveguide. When the input optical signal is of an optical pulse signal, the refractive index of the waveguide is relatively decreased during a lower level period of the pulse and is relatively increased during a higher level period of the pulse. In this manner, the refractive index of the waveguide is changed in accordance with a power level of the input optical pulse signal and the propagation constant of the waveguide is also changed. Therefore, the optical coupling condition between the two waveguides can be adjusted in a digital fashion in accordance with the lower and higher power levels of the input optical pulse signal.
According to an aspect of the invention, the input optical pulse signal having the first wavelength xcex2 which is to be converted is made incident upon the first waveguide together with a non-modulated optical signal having the second wavelength xcex2, and the propagation constants of the first and second waveguides are determined such that a propagation constant difference xcex94xcex2 between the first and the second waveguides when only the optical signal having the second wavelength xcex2 propagates along said first waveguide is smaller than a propagation constant difference xcex94xcex2 between the first and the second waveguides when both the input optical pulse signal having the first wavelength xcex1 and the non-modulated optical signal having the second wavelength xcex2 propagate along the first waveguide. Then, during a lower power level of the input optical pulse signal, the optical signal having the second wavelength is emitted from the second waveguide with a higher power level. However, during a higher power level of the input optical pulse signal, the optical coupling between the first and second waveguides is released and no optical signal is emitted from the second waveguide. That is to say, in this case, the output optical signal has a lower power level. In this manner, an inverted optical output signal having the second wavelength emanates from the second waveguide.
According to the invention, it is not always necessary that the two waveguides are constructed by the active waveguide, but at least one of the two waveguides upon which the input optical signal is made incident is constructed by the active waveguide. For instance, the first waveguide is constructed by the active waveguide and the second waveguide may be constructed by the passive waveguide. Furthermore, both the first and second waveguides may be constructed by the active waveguide
When the first waveguide is constructed such that a propagation constant difference xcex94xcex2 between the first and the second waveguides in a case that only the optical signal having the second wavelength propagates along said first waveguide becomes substantially zero, the waveguide length L becomes equal to the complete coupling length, and therefore the optical wavelength converter has a high conversion efficiency.
In a preferable embodiment of the optical wavelength converter according to the invention, said first and second waveguides are constructed by first and second semiconductor optical amplifiers, respectively which are formed on a same substrate. In this embodiment, the first and second optical amplifiers having substantially identical propagation constants can be manufactured on the same substrate by means of the well developed semiconductor manufacturing process.
In another preferable embodiment of the optical wavelength converter according to the invention, each of the first and second semiconductor optical amplifiers is formed to have the waveguide length within a range from 100 xcexcm to 5 mm. It is desirable that the waveguide length of the semiconductor optical amplifier is substantially identical with the coupling length. However, the coupling length could not be determined at will, but depends on, for instance the distance between the waveguides and the amount of injected current. If the waveguide length is not larger than 100 xcexcm, the two waveguides have to be coupled along a short distance, and thus the distance between these waveguides has to be decreased. It is apparent that such a structure could not be easily manufactured and the manufacturing yield might be decreased. Moreover, the amount of the injected current has to be increased. If the waveguide length is not smaller than 5 mm, the whole assembly might be large in size, and the second optical signal having the second wavelength xcex2 might be subjected to external influences and the control of the coupling condition could be performed only with difficulty. Therefore, according to the invention, the waveguide length is preferably set to 100 xcexcm-5 mm.
In another preferable embodiment of the optical wavelength converter according to the invention, a DC bias current source is connected across first and second electrodes of at least one of the first and second semiconductor optical amplifiers. By adjusting the DC bias current, the propagation constant difference between the two waveguides can be controlled. Although the two semiconductor optical amplifiers are formed on a same substrate using the well developed semiconductor device, it is sometimes difficult to make the propagation constants of the optical amplifiers identical with each other. In such a case, it is very effective to provide means for controlling the propagation constant difference. The refractive index of the waveguide can be adjusted by controlling the amount of an electric current injected into the waveguide, and therefore the propagation constant difference can be controlled by adjusting the bias current.